The present invention relates to a procedure for making photographic copies of photographic originals in a photographic color copying machine.
Today, photographic copying machines (photographic printers) are used in modern photographic laboratories to make color copies automatically from color negatives. Generally, the originals found on film strips in this equipment are scanned photo-electronically for the three basic colors of red, green, and blue in a measurement station of the photographic copying machine. The results of the measurement are evaluated and used for determining the required amount of exposure light for the copies. Then, the desired copies are made with the determined amount of exposure copying light in an exposure station of the photographic copying machine.
The procedures used for determining the amount of exposure light for the copies are based mainly on modifications of the basic principle described in U.S. Pat. No. 2,571,697. Generally, satisfactory color copies are obtained if the amount of exposure light for the copies is measured for the three basic colors in such a way that the color densities measured on the copies result on average in a neutral gray, regardless of the basic coloration of the original being copied. In this simple case, the color densities of the original that have been established for the entire original serve as a basis for controlling the exposure. This procedure, which is known as "integral correction," does not work if larger areas of the same color, namely so-called "color dominants," appear in the original, because then, for example, color-neutral areas on the original are reproduced on the copy in the color that is complementary to the dominants.
In the case of modem equipment, where the original being copied is scanned sectionally at a number of locations, attempts have been made to improve this procedure, for example, by not accounting for highly color-saturated areas in the original or giving only reduced weight to these regions in calculating the aforementioned mean values. Formulated generally, the required amounts of exposure light for the copies are determined on the basis of a color analysis of the originals, whereby the "color" of a measurement point is usually determined relative to the reference values, the so-called normal densities. Most expediently, the measured densities of color neutral regions of the original that are normally exposed are used as reference values. Another possibility exists in defining the reference values as the mean values of the three measured densities in the three basic colors for a larger number of originals.
In particular, depending upon the make, it is known that negatives have markedly different basic colorations. Therefore, it is necessary to determine a specific set of reference values for each type of film. The negatives used today are usually coded (DX-code, expanded DX-code), from which the type of film can be identified. As a result, it is easily possible to determine the given measured data and to code or adapt them on a constant basis. Another possibility is to determine the reference values for each film individually. For example, this can occur as a result of a (weighted) message about the measured values (measured densities) resulting from this film.
The procedure can be improved by using reference values that are not a function of a so-called mean density of any given measurement point. The mean density of a measurement point of the original, for example, is defined as the arithmetic mean of the three measured densities in the three basic colors of red, green, and blue. In this way, color defects of the original that are a function of the original can be accommodated. For example, this procedure is described in DE-A-29 12 130.
The outlined procedures for determining the color of originals (sections) can be viewed as simple standardization, having the objective of reducing the influence of the film-specific qualities on the color data used for the image analysis; these qualities can be a function of the type of film or also vary from job to job for a certain kind of film. It has been seen in practice, however, that the procedures based upon the determination of reference values do not provide the best possible measurements for image analysis for all types of originals being copied. This limits the reliability of the image analysis and is an important reason why satisfactory copying results are not always achieved.
Therefore, the invention is directed to creating a procedure for analyzing photographic originals being copied that provides color data for image analysis that is to a great extent independent of the film used. The procedure should make it possible to convert the color deviations in the original that were determined on the basis of image analysis into correction values for the exposure, which in turn takes into account the characteristics of the copying material. Another task that exists is to restructure the measured data in such a way that one of the coordinates in the standardized color region can serve as a norm for the lightness or "neutral density" of the original, whereas two other coordinates extend across a "color field" making it possible to analyze the pure color characteristics (color tone and saturation) of the original.